the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Projected changes of Active layer thickness over permafrost under 1.5~5.0 °C climate warming on the Qinghai-Tibet Plateau
Abstract. Permafrost degradation and active layer thickness (ALT) changes on the Qinghai-Tibetan Plateau (QTP) are caused by soil temperature increases under the continuous increase in global temperature. Therefore, the soil zero-degree layer can be used as an index to investigate the changes in permafrost and the ALT. The observed and projected permafrost and ALT were estimated by the summer soil zero-degree layer using soil temperature data from weather stations and the Coupled Model Intercomparison Project Phase 6 (CMIP6). The results revealed that the ALT is deeper in summer (e.g., July), indicating that the melting capacity increases. The CMIP6-simulated and observed soil temperatures are consistent in the vertical direction across the QTP, but the model results exhibit significant cold deviations. The average ALT on the Qinghai‒Tibet Plateau was approximately 3.75 m (range of 1.10~15.91 m) during 1961–1990 and increased to 5.77 m (range of 1.72~13.53 m) during 1991–2022, an increase of 53.9 %. The observed ALT were >6 m in the southeastern QTP area, such as east and south of Shigatse and Lhasa, where it was <3 m in the western Pamir Plateau, near Gaize, north of Qamdo in the northeast and east of Golmud, and other central and northern regions, where the values were 3–6 m. The ALT will continue to increase under the four Shared Socioeconomic Pathways (SSPs), especially when the radiative forcing level is high, and they will increase by 39.6 % under SSP5-8.5 by the end of the 21st century. The regional average of ALT will increase by 5.4 % for every 0.5℃ increase in global warming levels, increasing from 10.6 % to 47.1 %. A small change (e.g., <20 %) in the coverage area of the ALT will occur, decreasing from 1,424,735 km2 to 13,682 km2. However, the coverage area with a depth increase of more than 100 % will increase from 0 to 401,433 km2. The regions where the ALT slightly increases are primarily distributed in the northeast, east and southeast regions of the QTP, and the region where the ALT dramatically increases is in the western region, where there is concentrated permafrost.
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Status: open (until 12 Dec 2024)
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RC1: 'Comment on egusphere-2024-1169', Anonymous Referee #1, 08 Oct 2024
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Authors studied the thickness of soil zero-degree layer using soil temperature data from weather stations and 29 GCM of CMIP6. They tried to use the depth of soil zero-degree layer in July to estimate active layer thickness (ALT), and analyzed and projected the change of ALT under SSPs scenario. However, the author's concept of ALT is wrong, and the soil zero-degree layer depth in July cannot represent ALT. In the seasonal frozen soil area, soil are in a state of unfrozen from the surface to the underground, soil freezing process occurs only in the cold season, and the depth of the soil zero-degree layer is called the seasonally freezing depth, which generally occurs from September of the year to May of the next year. In permafrost areas, soil are frozen from the surface to a certain depth of the ground, soil thawing process will occur only in the warm season, the depth of the soil zero-degree layer is called the seasonal thawing depth, which generally occurs from May to September. Therefore, soil zero-degree layer depth is different for seasonally freezing areas and permafrost regions, not active layer depth.
The thickness of the layer of the ground that is subject to annual thawing and freeing in areas underlain by permafrost (Permafrost terminology from IPA). In general, the seasonal thawing depth in the permafrost region of the Qinghai-Tibet Plateau reaches its maximum in early September to late October. The maximum seasonally thawing depth is defined as ALT. In this paper, soil zero-degree layer depth is seasonally thawing depth in permafrost regions and seasonally freezing depth in seasonally freezing regions. Authors do not divided the permafrost areas and the seasonally frozen soil areas. ALT is mixed together to analyze and discuss. Therefore, results, discussions and conclusions given by the author are incorrect. In other words, the method developed by the author to estimate ALT by soil zero-degree layer is wrong.
Citation: https://doi.org/10.5194/egusphere-2024-1169-RC1
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